A new proposed class of Passive Optical Networks (PONs) are under development. These particular PONs may be collectively known as DPONs, for DOCSIS PONs. DOCSIS is the acronym for Data Over Cable Service Interface Specification, commonly known as the standard for cable modems. The new developments under development address the combination of PON technology and DOCSIS technology.
PONs are becoming increasingly popular as a way to deliver video, voice, and data services to homes and businesses. Meanwhile, normal cable TV systems send RF-modulated signals part of the way to the home on fiber optic lines, and then transfer the signals to coaxial cable typically a few miles from the home.
Opposite to normal cable TV systems, PONs, which are also known as fiber-to-the-home (FTTH) systems, can use fiber optic lines all the way to the side of the home. Conventional PONs are well-known to one of ordinary skill in the art, and include systems conforming to the current Broadband PON (BPON) (ITU-T G.983) standard, Gigabit PON (GPON) (ITU-T G.984) standard, and Ethernet (PON) (EPON) (IEEE 802.3ah) standard. The first two standards were produced under the auspices of the International Telecommunications Union (ITU), while EPON was produced under the auspices of the Institute of Electrical and Electronic Engineers (IEEE). All of these standards use so-called baseband (not RF modulated) data transmission, and all have an optional overlay for downstream RF broadcast video.
One problem with all of the standard PONs is that they do not work with DOCSIS data transmission, the data standard with which the cable TV industry is most familiar and most comfortable. Cable TV systems have constructed large control networks built around DOCSIS, and they have trained thousands of engineers in DOCSIS data transmission. Because of this, many Cable TV systems are interested in a PON standard that allows them to retain all of their DOCSIS (and RF video) equipment and experience, while offering FTTH, which is being demanded in many new home developments due to it's superiority in reliability, operating cost, and performance.
Accordingly, a new type of PON, herein called a DPON, for DOCSIS PON, has been proposed and is in the process of being commercialized. Several variations of conventional DPONs exist in the market. Most conventional DPON systems have the same common physical architecture of standard PONs, such as the afore-mentioned BPON, GPON, and EPON.
Meanwhile, DOCSIS, even in it's most advanced form, when compared to BPON, GPON or EPON, the system is restricted in its ability to transmit high speed data. However, some cable operators are willing to put up with this limitation in order to continue use of the DOCSIS data transmission standard so that existing legacy equipment can be used. Such use of legacy equipment can keep operating costs for cable operators low compared to purchasing new equipment for supporting newer standards.
Referring now to FIG. 1, this Figure shows the physical architecture of a conventional DPON network, including the terminations at the head end and at the home. The conventional art referred by this disclosure does not provide a publication date to establish that it is published more one year prior to the date of the earliest claimed priority date of instant application. Further, the conventional art does not show that the subject matter was in public use or on sale in this country, more than one year prior to the earliest claimed priority date of the present application.
Moreover, “conventional art” is referred to in this writing rather than “prior art” to provide a basis of comparison for the claimed invention which is an improvement over this conventional art. Therefore, the conventional art referred to in this disclosure is not “admitted prior art” as defined under local rules certain jurisdictions.
Referring back to FIG. 1, the DPON 111 which includes the optical transmitter 101 extends from the Wave Division Multiplexer (WDM) 103 to the Optical Network Terminations or Terminals (ONTs) 105. ONTs 105 can also be referred to Optical Network Units (ONUs). The WDM 103 can join optical signals together and can split optical signals apart. The WDM 103 can function as a wavelength-sensitive multiplexer. The WDM 103 can comprise optical filtering devices such as etalons or stable solid-state single-frequency Fabry-Perot interferometers in the form of thin-film-coated optical glass.
The optical splitter 104 is a 32-way splitter, though other split ratios are possible. Physically, the DPON 111 looks about the same as a true PON (e.g., GPON or EPON). The difference is that all signals on the fiber plant are carried in RF format, which is more familiar to cable operators. The downstream signal is produced using a conventional 1550 nm RF (analog) optical transmitter 101. The downstream optical signals can contain video information which can be converted to the electrical domain by the ONT or subscriber optical interface 105 and processed by each set top box 107. Each set top box 107 can be coupled to a television 706.
For upstream transmission of RF from the home, each ONT 105 includes an optical transmitter, which is turned on only when a set top box 107 or cable modem 108 to which the ONT 105 is attached, transmits a signal. Upstream signals 109 from a set top box 107 or upstream signals 110 from a cable modem 108 are usually RF signal modulated onto either a 1310 nm optical carrier, or an optical carrier at some other wavelength, for example 1540 or 1590 nm. Each set top box 107 and each cable modem 108 of a particular house may be coupled to the same optical network terminal (ONT) 105 through an RF splitter 106. Each set top box 107 can use protocols such as the American National Standard Institute (ANSI)/Society of Cable Television Engineers (SCTE) Standard 55-1 (formerly Digital Video Subcommittee-DVS 178) SCTE 55-2, and/or DOCSIS Set-top Gateway (DSG).
The higher wavelengths for upstream signals 109, 110 are typically used to keep the conventional GPON/EPON wavelengths of 1490 nm and 1310 nm open, so that a conventional PON may be overlaid on the DPON. We shall refer to a 1310 nm upstream wavelength, understanding that the upstream optical carrier could be at some other wavelength. The systems described in the disclosure are typically not a function of which upstream wavelength is chosen.
In general, the upstream signals 109, 110 will usually comprise burst signals from two types of sources in each home: one source can include a DOCSIS cable modem 108, and the other source can include an RF return from a conventional set top box 107 using either the SCTE 55-1 or 55-2 standards, or any other type of RF return protocol. One serious problem is that there is no practical way to prevent both a DOCSIS modem 108 and a set top box 107 from transmitting at the same time (109 and 110). If one of each in different houses which are coupled to the same PON 111 try to transmit simultaneously, then two upstream transmitters 107, 108 will be on at the same time. The collision of the upstream signals 109, 110 at the optical detector 102 can cause unacceptable noise if the two optical wavelengths for the upstream signals 109, 110 are close enough.
It is noted that the condition of two set top boxes 107 or two cable modems 108 within the same house or in different houses transmitting at the same time is not considered. The set top boxes 107 and the cable modems 108 each have its own control system in the head end that will usually prevent two like devices (modems 108 or set top boxes 107) from transmitting at the same time. However, there is no practical way to tie the set top control system to the cable modem control system, such that one set top box 107 and one cable modem 108 cannot transmit at the same time. The possibility that the set top 107 and cable modem 108 in the same house will transmit at the same time is generally not a concern, because if that happens, it is likely that both will get through.
In addition to the problem of potential collision of upstream signals originating from separate houses, another potential problem of conventional systems is the need to support DOCSIS 3.0, which may have four RF channels of complex modulation. This requires a relatively high degree of linearity in the optical transmitter, and requires relatively high optical power to transmit the upstream signal. The SCTE 55-1 and -2 RF Return signals are QPSK modulation at lower bandwidth, and hence do not require as much optical power to transmit them to the head end.
Accordingly, there is a need in the art for a method and system for supporting DOCSIS over a PON. Particularly, there is a need in the art for reducing or eliminating the possibility of upstream return signals originating from a set top box 107 of first subscriber's premises from colliding and reducing reception of cable modem return signals originating from a cable modem 108 from a second subscriber's premises. There is a further need in the art for more robust optical transmitters 101 to support DOCSIS signals which may employ several RF channels of complex modulation.